After upgrading to SG Pro 3.0 with an option for individual event gains I decided to lower my ASI1600 139 gain settings as my plan was to use the gain 75 with Nebulae and gain 0 with Galaxies, both with offset 10. Yesterday evening being the first "clear" in two weeks I continued my journey with my old friend Monkey Head, starting with 5 minutes exposures and gradually increasing the integration time all the way up to 10 minutes because the mean ADU's stayed at low 300's and I didn't get any left-side separation on Histogram.

Any ideas why these ADU's don't get any higher even when I'm doubling the exposure times? With the gain 75 I was hoping to get 600 - 700 mean ADU's but as you see I didn't come even close. I'm guessing my Bortle skies are 4.5 - 5 so it's not like I'm doing this in the middle of the Kalahari Desert. And don't get me wrong, I love these longer exposures / less subs and rather process 60 pcs of 10 min subs for 10 hours than 200 pcs of 3 min subs. Just curious why I cannot get any left-side Histogram separation.

Great AstroCat. Like said, I don't mind going even longer than 10 min as long as the CEM60 keeps up with those integration times (which I have no doubt). I was assuming a slight increase to these times when moving from ZWO filters to AstroDons' but nothing like this. It's just that all posts of ASI1600 talk about short exposure times.

I suspect that you will need far longer exposures than 10 minutes plus. I routinely use 30 minute exposures on my Atik 460 when shooting NB so I think 10 minutes would be just the start. It would be very difficult to saturate the sensor even for exposures totalling hours.

Great AstroCat. Like said, I don't mind going even longer than 10 min as long as the CEM60 keeps up with those integration times (which I have no doubt). I was assuming a slight increase to these times when moving from ZWO filters to AstroDons' but nothing like this. It's just that all posts of ASI1600 talk about short exposure times.

- Sal

A lot of the talk around the short exposure times is to ensure you don't blow out any stars. There was a thread with a bunch of short exposure times listed for different levels of light pollution with exposure times down to 6s for LRGB and (I think) 30-90s for narrowband. But all those times were based on not blowing out the stars...not necessarily for getting the histogram moved to the center to better expose the subject. So there was a ton of debate about the validity of having such short exposures to protect the stars if it meant you would end up with 20,000 exposures you'd have to process for a single final image.

I don't see anything wrong with your times so long as you're getting results you like. I don't have the Astrodon's but I did notice even on the ZWO filters the difference between 5 and 10 minutes for me was not noticeable on the histogram. From what I've read, you need to get into the 30 min range before it really starts to move. But I have no personal experience with that long of an exposure.

Well, with narrowband you are not going to see massive changes in ADU values like you would with broadband images. You are blocking so much LP and other business that the additional exposure time does not act the same. Instead of caring about that, how did the images come out?

Any ideas why these ADU's don't get any higher even when I'm doubling the exposure times? With the gain 75 I was hoping to get 600 - 700 mean ADU's but as you see I didn't come even close. I'm guessing my Bortle skies are 4.5 - 5 so it's not like I'm doing this in the middle of the Kalahari Desert. And don't get me wrong, I love these longer exposures / less subs and rather process 60 pcs of 10 min subs for 10 hours than 200 pcs of 3 min subs. Just curious why I cannot get any left-side Histogram separation.

As usually - thanks,

- Sal

What is your scope? F-ratio and aperture will play a role here. Aperture will affect how quickly the stars saturate (bigger -> faster star saturation), and f-ratio will affect how quickly the extended objects saturate (smaller f-ratio -> faster extended saturation).

If you are using a high f-ratio scope, then longer subs may be necessary.

Using extremely long subs is non-optimal with most CMOS cameras. The amp glows tend to accelerate, get brighter faster with longer exposures (they often have non-linear behavior in time), so exposing most CMOS cameras much longer than 10 minutes is usually not necessarily going to net you better results than stacking more 10 minute subs (you often just end up trading additional dark signal noise for read noise, and sometimes the read noise from many frames is lower).

That said, the f-ratio will ultimately be the determining factor here...a slow f-ratio may force you to expose longer than 10 minutes if you aren't swamping the read noise enough. As for what that actually means...note that, with very low read noise (i.e. 2e- at Gain 76 on the ASI1600), to swamp the read noise squared by 10x, you only need 40e-. Compare that to the say 250e- you might need with a camera that had 5e- read noise, or the 810e- you would need with a camera that had 9e- read noise. At 40e-, you are often not going to see much "separation from the left", even though you may be plenty sufficiently exposed.

Jon I was talking about AT115 which is a f/7. I'm happy if I can keep taking 10 min subs. I think it's kind of best of both worlds (long enough that I don't have to integrate hundreds of subs, yet short enough that if something goes wrong, I don't lose half an hour every time).

rockstarbill I think the images look pretty decent. My backfocus is still a bit off as I'm waiting for a shorter extender but my HFR's were under 2 so overall I'm not complaining...